Material food first changes into the one who eats it, and then, as a consequence, restores to him lost strength and increases his vitality. Spiritual food, on the other hand, changes the person who eats it into itself. Thus the effect proper to this Sacrament [of the Eucharist] is the conversion of a man into Christ, so that he may no longer live, but Christ lives in him; consequently, it has the double effect of restoring the spiritual strength he had lost by his sins and defects, and of increasing the strength of his virtues.
Saint Thomas Aquinas
Contemplative in the Mud 